Page make-up systems are a well established art. In particular it is known to form a page to be printed from a number of elements which may be text, image or line-work elements. Such elements may in general overlap and systems exist which choose to treat portions of one of the overlapping elements as being “transparent” with respect to the other element, i.e., portions through which underlying elements can be seen.
In PCT publication number WO 95/02224, the assignee's of the present application described a system in which non-overlapping elements could be effectively combined on-line to form a page. In this publication, elements are stored in bit-mapped (half-toned) form together with a control database which describes the position and extent of the elements. The control database is also described as including a description of positions and extents of blank regions between the elements.
In use, the device described in WO 95/02224 constructs a page on a line by line basis by sweeping through the element memories and writing the bit map from the element memories (and zeros for the spaces) on a line by line basis into a page memory.
This system, while it does allow for the efficient and quick placement and writing of the elements into a page, with great flexibility as to placement within the page, does have a number of limitations. In particular, closely spaced non-rectangular elements, such as, for example, italic letters, require decomposition of the letter into a large number of elements, each of which must be addressed and separately read into the printing memory. This reading requires a large number of time consuming switches between non-sequential memory locations in order to write each line. Another example of such a problem is the situation in which text is overwritten onto an image. The described system requires that the image and text be decomposed into a large number of non-overlapping rectangular elements in order for the image to be handled.
Furthermore, the system of WO 95/02224 teaches that an entire line of the page be written into memory at one time. This generally requires that the spaces (the blank areas) also be written into the memory. Thus, if for example, a line contains portions of a large number of elements and spaces then the number of switches between non-sequential memories in which the elements are stored may be so large as to severely limit the throughput of the system.
Furthermore, since elements have a varying extent and position in a cross-scan direction, it is necessary to scan the entire page prior to printing and the use of buffers is thus made more difficult and expensive.
Finally, since the images, especially in bit mapped (half-toned) form, are very large and the number of possibilities of bit mapped combinations of elements may also be very large, saving the possible bit maps may be very memory intensive and the printing memory requirements for a relatively small job may be large. It particular, in a card printing embodiment of the PCT publication, in which italic script names are printed in white overlaying an image, millions of possible combinations of elements must be provided to allow for the wide variety of possible names to be printed.